The invention relates to a faceplate assembly for an imaging device and, more particularly, to a faceplate assembly having a faceplate flange with integral gauging means which permits a faceplate member to be sealed parallel to the faceplate flange and at a predetermined distance therefrom.
In imaging devices, such as camera tubes and image intensifier tubes, a lens focuses the image of a scene onto an input faceplate of the device. Many camera tubes, such as silicon intensifier tubes, image isocons, intensified vidicon tubes and intensified charge coupled devices, utilize a photoemissive cathode as a light sensor on the interior surface of the input faceplate. Photons, comprising the image of the scene, pass through the input faceplate of the device into the photoemissive cathode. The incident photons produce photoelectrons which are accelerated to a target or to a charge coupled device. In order to minimize distortion of the ultimate image, it is necessary that the image of the scene be focused over the entire input faceplate of the device. This requires a flat, substantially parallel input faceplate.
Typically, the input faceplate comprises a plano-concave fiber optic which is frit sealed to a faceplate flange to form a faceplate assembly. One such structure is shown in U.S. Pat. No. 4,243,905, issued to van Geest et al. on Jan. 6, 1981. Frequently, because of variations in the thickness of the frit layer, the faceplate is slightly tilted relative to the faceplate flange. The tilting of the faceplate is undesirable because the image of the scene is not sharply focused over the entire faceplate area. Such tilting results in a decrease in resolution of the ultimate image. Additionally, the faceplate tilt introduces spacing variations within the tube which further degrade the resolution. The resolution degradation is most pronounced in diode-type tubes in which the electron-optical focusing is optimized for a predetermined spacing between the interior surface of the faceplate and the anode.
In tubes in which the input faceplate is not sealed parallel to the faceplate flange, it is necessary to grind and polish the exterior surface of the faceplate, after tube fabrication, to obtain the necessary parallelism for proper scene focusing on the input faceplate. Such a grinding and polishing operation is time consuming, expensive and occasionally results in the loss of a finished tube due to mishandling or the generation of particles within the tube as a result of the vibration of the grinding and polishing operation.